In recent years, there has been developed a nondestructive sensing technique using an electromagnetic wave (30 GHz to 30 THz) ranging from a millimeter wave to a terahertz (THz) wave. Techniques under development in the application field of the electromagnetic wave having this frequency band include a technique for carrying out imaging using a safe radioscopic inspection system in place of an X-ray system, a spectroscopic technique for obtaining an absorption spectrum or a complex dielectric constant of the inside of a substance to evaluate a coupling state of atoms, a technique for analyzing a biomolecular, and a technique for evaluating concentration or mobility of carriers.
JP10-104171A discloses a THz generating means suitably employing a photoconductive element in which an antenna also serving as an electrode is provided on a photoconductive film deposited on a substrate. A detector 132 is shown in FIG. 10 as an exemplary structure of the photoconductive element. A substrate 30 has, for example, a silicon-on-sapphire structure treated with radiation, in which a silicon film as a photoconductive material is deposited on a sapphire substrate. Generally, LT-GaAs grown at a low temperature on a GaAs substrate is also often used as a photoconductive film. A dipole antenna 138 formed on a surface includes a pair of dipole feed lines 138a and 138b and a pair of dipole arm portions 139a and 139b. An optical pulse is condensed at a gap 133. When a voltage is applied across the gap, a THz pulse is generated. When photocurrent is detected without applying a voltage, a THz pulse can be detected. A substrate lens 136 serves to couple a slab mode (substrate mode) of an electromagnetic wave confined in a substrate 130 to a free-space radiation mode and also serves to control an angle of radiation in the electromagnetic wave space-propagation mode.
The foregoing illustrates by way of example that a photoconductive element is used alone to propagate an electromagnetic wave into space. On the other hand, Applied Physics Letters, vol. 80, no. 1, Jan. 7, 2002, pp. 154-156 and Applied Physics Letters, vol. 84, no. 12, Mar. 22, 2004, pp. 2049-2051 disclose a small functional device in which a semiconductor film functioning as a photoconductive element and a transmission path for transmitting a generated electromagnetic wave are integrated on a single substrate. This device has such a structure that a thin film of only an epitaxial layer of a photoconductive element made of LT-GaAs is transferred on a portion of a high frequency transmission path formed on a substrate. According to Applied Physics Letters, vol. 80, no. 1, Jan. 7, 2002, pp. 154-156, a microstrip line is formed on an Si substrate with an insulator resin interposed. A gap is formed at a portion of the line. An LT-GaAs thin film is arranged only at the bottom of the gap. On the other hand, according to Applied Physics Letters, vol. 84, no. 12, Mar. 22, 2004, pp. 2049-2051, coplanar strip lines are formed on a quartz substrate. An LT-GaAs thin film is partially arranged to extend across a gap between the two lines.
Each device performs driving so that a laser beam is applied, through spatial propagation, to the gap of the metal line from the surface side of the substrate to allow a generated THz electromagnetic wave to propagate through the line.